Understanding Smart Sensors - Nomads.usp

Mechatronics and Sensing Systems 255Each outputturns offindependently5 A/div10 msFigure 11.3 Independent thermal shutdown of an eight-output smart-power IC.difficult condition to detect. Soft shorts require overtemperature sensing toprotect the IC from destructive temperature levels.The overtemperature condition sensed by the power IC may mean thatthe device turns itself off to prevent failure in one case; in another situation, afault signal provides a warning to the MCU, but no action is taken, dependingon the fault circuit design. The remaining portion of the system is allowed tofunction normally. With the fault conditions supplied to the MCU, an orderlysystem shutdown can be implemented.Temperature sensing also has been integrated into a standard powerMOSFET process. The power MOSFET process was chosen instead of a powerIC process because a large die size was required to meet the power dissipationrequirements of a very cost sensitive automotive application. A minor modification(the addition of a single masking layer) to a production power MOSFETprocess allows both sensing and other protection features to be integrated.The thermal sensing is provided by integrated polysilicon diodes. Bymonitoring the output voltage when a constant current is passed through thediode(s), an accurate indication of the maximum die temperature can beobtained. A number of diodes are actually provided in the design. A singlediode has a temperature coefficient of 1.90 mV/°C. Two or more diodes can beplaced in series if a larger output is desired. For greater accuracy, the diodes canbe trimmed during the wafer probe process by blowing fusible links made frompolysilicon.The response time of the temperature-sensing diodes is less than 100 ms,which has allowed the power device to withstand a direct short across an